Concrete and mortar structures can degrade and even fail due to aging, freeze/thaw cycles, deicing salts, shrinkage cracking, and other environmental effects. Reducing corrosion in reinforced steel concrete is a major challenge. Corrosion of reinforcing steel in a concrete structure is caused by natural environmental effects, such as reactions with chloride ions from salt water and deicing salts applied to the surface of the concrete. Chloride ions migrate through the concrete and corrode the embedded reinforcing steel. The ability of chlorides to migrate through the concrete is referred to as the chloride ion permeability of the concrete. In low permeability concrete, the rate at which chloride ions come into contact with the steel is low, which increases the stability of the embedded steel.
The permeability of unmodified, ordinary Portland cement (OPC) ranges from 3,500 to 12,000 coulombs as determined by ASTM C 1202. Permeability is generally reported at the age of 28 days. Table I below lists the chloride-ion permeability of various concretes. The abbreviation w/c is the water to cement ratio.
TABLE ICharge Passing Chloride Permeability(Coulombs)Typical Concrete TypeHigh>4000High w/c ratio (>0.60)conventional OPC concreteModerate2000 to 4000Moderate w/c ratio (0.40-0.50) conventionalOPC concreteLow1000 to 2000Low w/c (<0.40) conventionalOPC concreteVery Low 100 to 1000Latex-modified concrete,internally sealed concreteNegligible<100Polymer-impregnatedconcrete, polymer concrete
Beginning in the 1960s or earlier, polymer latex was added to Portland cement to provide a low permeability concrete, as described in U.S. Pat. Nos. 4,086,201, 3,043,790, and 3,895,953, incorporated herein by reference. As used in concrete, latex is known for its plasticizing and adhesion properties and can improve tensile strength, compressive strength, and flexural strength, while also reducing permeability by filling in the large porosity of OPC matrices. Latex modified concrete has been used for bridge deck overlays, patching, and full depth pavement repairs, as it tends to be more durable in environments involving intensive free-thaw cycles and exposure to high salt concentrations.
Notwithstanding these advantages, latex modified concrete remains in limited use. The high cost of the latex polymer is a significant drawback. It is generally provided in two to three component systems, which complicates storage and handling. Indeed, storage and handling is a major factor in the cost of latex modified concrete. Since latex modified concrete has a short working time, it must be placed soon after mixing with OPC, and it cannot be transported over long distances. Consequently, a special mobile vehicle is required to store and dispense the two or three part latex blend into the concrete mixer at or very near to the job site. Typical latex modified concrete is 4-7 times more expensive than OPC concrete and is therefore only used in harsh environments. Hence, improved, easier to handle low permeability cements, mortars and concretes are needed.
While low permeability, traditional OPC concretes have been successfully manufactured, they still exhibit the undesirable characteristics of OPC: long setting times, low durability and high shrinkage. Rapid setting cements, such as those based on calcium sulfoaluminate as described in U.S. Pat. No. 3,860,433, incorporated herein by reference, have short setting times which allow re-opening of a bridge within hours of its closure instead of days.